Handle blank mobiledoc in editor

[bantic]

This removes some code that previously forced an empty mobiledoc to
have a single empty section in it. A mobiledoc can now be truly empty and
the editor will display it and respond to events correctly.

Added Post#isBlank and in the keydown listener, if the post is blank the
editor will insert a new empty markup section under the cursor.

Now that it is possible to have a fully empty mobiledoc, the css for showing
the placeholder will apply again. We currently stop a "backspace" keystroke if
it is at the beginning of the first markup section, so it is not possible to use
the keyboard to make a post completely blank, but it's possible to do so by removing
a card from the editor if the card was the only thing there.

Updated the demo.js with a removable card demo and an empty mobiledoc demo
so that the two scenarios described above are viable to test and see the
placeholder text rendered.

Added some additional handling for the embed intent (the "+" icon that shows up)
to ensure that it does not throw errors and that it does show up when the
editor is showing an empty mobiledoc.

• Rename Section methods semantically
• add "simple-card" demo, with "remove" button
• Clean up cursor detection and embedIntent
• Add Position.emptyPosition and Range.emptyRange
• allow mobiledoc to have 0 sections
• add Post#isBlank
• Add tests for typing in blank mobiledoc
• Tests for removing the last item (e.g., card) in a mobiledoc
• Ensure that replaceSection works when section is undefined
• Add empty mobiledoc to demo

fixes #125 #35 #71